One type of shoulder joint prosthesis is called a humeral head “resurfacing” implant, and another is a humeral head “replacement” implant. Humeral head resurfacing is a conservative approach to humeral head arthroplasty and is accomplished by using a dome-shaped, thin wall shell structure to resurface the humeral head. The former implant has a fairly short central stem to achieve fixation to the humeral bone and to preserve most of the existing bone. In humeral head replacement, the entire humeral head is cut off during surgery, whereas the humeral head replacement implant has longer medullary stem to fix the implant to the humeral bone. FIGS. 1A, 1B and 1C illustrate two such implants representative of those presently in use.
As background information, the shoulder joint is formed by the head of the humerus articulating with a shallow socket called the glenoid. The glenoid is located on the lateral margin of the scapula, as shown in FIG. 2. Both the humeral head resurfacing and replacement implants can be used in total joint replacements or in hemi joint replacements. Total joint replacements typically include a polyethylene socket that replaces the glenoid as shown in FIGS. 3A and 3B.
More than half of the shoulder joint arthroplasties done in the US are hemi arthroplasties because the exposure to the glenoid is difficult; moreover, the bone mass comprising the glenoid is so limited that a glenoid replacement part may often loosen. In addition, humeral head replacement implants are more commonly used than humeral head resurfacing implants.
Presently, humeral head implants (both humeral head replacement and resurfacing implants) used in total and hemi joint replacement are made of Co—Cr alloy. However, it is now recognized that Co—Cr alloy is damaging to joint tissues (cartilage and bone) and is a shortcoming of such a hemi-arthroplasty. Pyrocarbon has been shown to be much less damaging to native joint tissues (cartilage and bone); thus, it is a better hemi-arthroplasty material than metal or ceramics, such as aluminum oxide or zirconia. Pyrocarbon is a brittle material and as such is not as strong and tough as Co—Cr alloy; thus, designs of pyrocarbon humeral head implant devices that meet the strength performance requirements are desired.